A big unsolved problem is the processing of large amounts of fine-particulate metal-containing material, for example of one million tons per year. Until today, the charging, most often blowing, of fine-particulate material into metallurgical melting furnaces, for instance electric furnaces, has been limited only to the processing of relatively small amounts, whereby the fine-particulate material accrues as breeze or waste. Currently, fine-particulate metal-containing material is foreseen merely as an addition to other metal-containing material being charged into the melting furnace in lumps or in the form of pellets or briquettes, respectively.
The plant-related reliability of the blow-in plants used for charging the fine particles is small, and the operating and maintenance costs are relatively high. It is not possible to supply the fine particles to the melting furnace, for instance, via a simple chute, since, due to the gas flow conditions prevailing in the melting furnace caused by low pressure, the fine particles would immediately be withdrawn from the furnace space through the suction means with which every melting furnace is provided. In order to avoid that problem, so far either the ore dusts have been processed to ore pellets in pelleting plants prior to being reduced or the reduced fine-particulate material has been processed to briquettes in hot briquetting plants and thus they have been rendered applicable for the conventional charging devices of melting furnaces. Those plants, however, do bring about very high investment costs.
Dusts of melting furnaces such as they accrue, for example, during the production of steel also have to be pelletized at high costs prior to being reused in a reduction aggregate, provided that they can be recycled at all and do not have to be dumped. It is not possible to directly use the dusts for reutilization purposes in the steel production.
All sponge metal products end up being hot after production, sponge iron, for example, with a temperature of more than 600° C. For a long time, it has been the wish of steel mill engineers to use that heat immanent to the charge material. However, due to equipment-related difficulties (pneumatic conveyance) or logistical problems (transport of containers), that could not be realized. The energy yield obtained by the supply of 100% sponge iron of a temperature of approximately 600° C. would amount to, for instance, more than 100 kWh/t liquid steel, which, so far, has been impossible to be made use of.
In order to prevent the directly-reduced fine particles from becoming reoxidized and in order to blow the fine particles into the melting furnace, moreover, large amounts of inert gases are necessary, with the costs for those inert gases rendering those methods more expensive.
When melting down metal in an electric arc furnace, great energy losses occur which have been caused by the significant amounts of energy being withdrawn by the hot off-gas and in the following by the wall and lid cooling of the furnace vessel and the cooling of the hot gas duct. So far, it has not been possible to even only partially reuse those large withdrawn amounts of energy. A method which would render that feasible could have other significant advantages with regard to its economic efficiency.
In a method according to the initially described kind, known from EP 0 134 852 A1, sponge iron particles having a great amount of fine-particulate material are fed into a melting furnace, whereby the feeding of the sponge iron into the melt is preformed in a gravitative manner via one to two inerted storage vessels by means of screw conveyors.
That, however, brings about the disadvantage that, due to the mutual reaction occurring between the sponge iron and the components of the slag during melting, boiling reactions and gas formations take place, which lead to gas flow conditions in the interior space of the melting furnace which counteract strongly to an introduction via gravity of the mostly fine-particulate iron particles and entrain the same so that the output is impaired extremely. Furthermore, it can be regarded as a disadvantage that a substantial wear of the mechanical conveying device expanding immediately into the metallurgical melting furnace (in its high temperature region) is the result.
It is known from EP 0 462 713 A1 how to charge iron-containing particles by the aid of a feeding device designed as a screw conveyor or a pneumatic conveying system. Hereby, a conveying tube of the feeding device expands obliquely into the high temperature region of the furnace, optionally partially into the slag, via an opening in the sidewall of the melting furnace.
Besides the gas flow conditions also occurring therein and having a negative effect on the conveyance, increased equipment-related expenditures and expenditures with regard to the technical operation are necessary in order to attain a high charging level of the material to be charged in the described manner, whereby also in that case the strain on the mechanical components of the feeding device occurring during the charging of hot particulate material represents a problem.
According to DE 36 21 323 A1, the supply of the metal-containing material into the melting furnace is carried out through the channel of a hollow electrode, which simultaneously supplies energy for melting the metal particles and the slag formers and for maintaining a metal bath.
The main disadvantage of that method consists in that the cross section of the electrode and hence the diameter of the channel are subject to a limitation depending on the current density necessary for the melting. Therefore, it is not possible to increase the dimensions of the electrode to any desired extent in order to obtain a higher charging portion of the metal-containing material. That way of supplying the metal-containing material, therefore, does not allow the use of the method for the production of steel to an extent customary today, since in doing so it would not be feasible to melt the amounts of reduced iron necessary for an efficient steel production.
A method of the initially described kind is known from WO 99/18245 A. Herein, by means of one or several lances, fine-grained, directly-reduced iron, optionally in the hot state, is fed into the foamed slag maintained in an electric arc furnace having vertical electrodes and is melted. The supply may be carried out merely by gravity according to WO 99/18245 A but also by means of a conveying gas.
However, that method has the drawback that by means of the lance only a small amount of sponge iron (DRI) can be produced, since the interior diameter of lances usually does not exceed 100 mm. However, when using several lances in order to increase the conveying capacity, it is difficult to uniformely supply those with conveying material.
That method has the further disadvantage that an introduction of charging material into the energetic centre of the furnace is not feasible, which, in case of higher conveying rates, results in the clogging of the lances, since it is not possible to melt the conveying material quickly enough. If several lances are arranged around a central D.C. electrode, that results in the material introduced via the separate lances being melted in different ways, since the electrode's electric arc forming the energetic centre of the furnace burns against the furnace bottom with a deflection against the horizontal line of about 5°. Since, furthermore, the electric arc only has small spatial dimensions, there is a substantially larger energy supply at one particular circumferential spot of the electrode than at other circumferential spots. Hence, with the concentric arrangement of the lances according to WO 99/18245, it is impossible to introduce the entire fine-grained iron material into the energy centre
In accordance with a further embodiment according to WO 99/18245, the electric arc has three A.C. electrodes, which, at uniform intervalls from each other, are arranged on a circle in the interior of the furnace. Inside the electrode circle, three lances provided for supplying the material are arranged concentrically. Apart from the practically existing lack of space for the lances inside the electrode circle, that lance arrangement also has the problem that melting takes place nonuniformely, since, due to their inherent resistance, the electric arcs burn to the outside and hence no energetic centre is formed inside the electrode circle.
From DE-A1-197 44 151 it is known to surround, in an electric arc furnace run by direct current, a centrically arranged electrode that projects vertically into the same by blowing-in lances, these lances projecting as far as into the slag layer. For alternating current, three electrodes that project vertically into the electric arc furnace are provided, between which three charging lances are arranged.
From DE 196 08 530 A, a method for the treatment of steel in an A.C. electric arc furnace having three electrodes arranged on a circle is known, wherein solids to be charged into the furnace are blown in by means of CO2 via a lance below the slag surface in the region of the electrodes.
Due to the electric arcs directed towards the outside in an A.C. furnace, no formation of an energy centre takes place. Also with that prior art, the solids are not charged into an energetic centre, which does not render feasible any higher conveying rates. Another disadvantage of that method is the expenditure for the pneumatic conveyance of the solids via the lances into the furnace.
According to U.S. Pat. No. 5,946,339 A, via a supply tube, DRI and fluxing agents are charged into an electric arc furnace having two electrodes, which are arranged at a distance from each other, so that, between them, the charging material falls into the slag located in the furnace. Thereby, a debris cone forms below the tube opening not dipping into the slag.
Due to the distance between the electrodes necessary for introducing the charging material, the energy is not centred at the spot where the solids are introduced, which makes a quick melting of the conveying material impossible.
From U.S. Pat. No. 2,894,831 A, an electric melting furnace is known which has two inclined electrodes serving for melting reduced iron powder, whereby, centrally above the melting furnace, a shaft is provided in which a material column standing of the furnace bottom is formed, by which shaft the electrodes are forcibly supplied with material to be melted.
However, the great distance of the only very slightly inclined electrodes caused by the shaft avoids the formation of an energy centre in the region of the material charge and hence slows down the melting process.
An electric arc furnace for melting down iron carriers, such as scrap, is known from EP-A1-0 663 450, a central charging shaft being provided above a furnace interior space. This charging shaft ends at the furnace covering and serves as a pre-heating shaft for scrap, wherein, for the purpose of pre-heating the iron carriers, off-gases of the interior space of the furnace via a gas-permeable cut-off device at the lower end of the charging shaft flow into the same and leave the charging shaft at the upper end. Fine-grained charging material is additionally charged via bottom-belt nozzles, lances or hollow electrodes.
A charging means for an electric arc furnace for charging scrap, in particular car scrap, is known from WO 93/13228. This charging means has a charging tube which may be raised and lowered, projecting into the interior space of the electric arc furnace. Raising and lowering of the lower end of the charging tube serves the purpose of being able to adjust the diameter of the scrap column formed by the scrap packages. Charging is effected out of center and at a quite large distance from the electrodes projecting vertically into the interior space of the electric arc furnace.
If large amounts of fine-particulate metal-containing material are to be processed, one is therefore currently forced to render the fine particles lumpy in investment intensive plants for pelletizing prior to reducing or briquetting after reducing in order to obtain the amounts necessary for an efficient steel production, whereby, however, the advantage of small raw material costs as opposed to lumpy ore is lost.